My transmitting loop started life as a full-size G5RV
antenna in inverted-vee format (basically, a centre-fed inverted-vee dipole, overall
length of 34 m, fed with 13 m of feeder). For 136 kHz, I joined the far ends of the
dipole together with some thick multi-strand cable to form a sort of top-fed delta loop
having an overall perimeter of about 65 m. The base is about 1.7 m above ground
level (AGL), and the apex is about 11 m AGL.
At the shack end, the feeder is connected to my Loop Antenna Tuning Unit (Loop ATU) which comprises a
capacitive matching network (3 capacitors) and a toroidal 4:1 balun.
As with any loop, the key is to make it as big as possible (to enclose the greatest
possible area), and to use the largest possible conductor size (to reduce ohmic
losses). The resistance of my loop used to be 1.7 ohms - much of this being due to a
9 m length of 300 ohm balanced feeder used as part of the 13 m feeder section. In
August 1999 I replaced the 300 ohm feeder with two lengths of 13 ampere, twin-cored mains
cable (with the conductors of each cable connected in parallel). This reduced the DC
loop resistance from 1.7 to 0.8 ohms and, at 18 W RF input to the Loop ATU, the current in
the loop increased from 2.3 A to 2.5 A. This implies that the overall loop impedance
(including the components in the ATU), reduced from 3.4 to 2.9 ohms.
Factors affecting the overall loop impedance appear to be:
- the DC resistance of the loop;
- the effective resistance of the matching components (i.e. C1; C2; C3 in the Loop ATU);
- the 'skin effect' of the conductors at 136 kHz.
On transmit, my best DX using this antenna has been G3KEV (about 300 km).
Dave G3YMC has had even more success with his transmitting loop. Although Dave's
loop is much smaller than my antenna, he has already worked 4 countries on 136 kHz, using
just 40 W of RF! If you're short of antenna space, you've got to see Dave's web site
for more information:
In views of my recent experiments with vertical antennas, André Kesteloot N4ICK has
asked me, "are you deserting the loop fraternity? Have you decided that there
was more power to be radiated from a vertical?". A good point indeed. I
have replied as follows:
"I still use the loop. Firstly, it is an excellent receiving antenna for my QTH
(a) all loops of this sort have deep nulls perpendicular to the plane of the antenna.
Because the loop is running east-west, the null to the south means that the noise
sidebands from the Loran transmitter located in France are inaudible on the loop (the
noise sidebands are S6 on my experimental vertical antenna); and,
(b) unlike unbalanced antennas, the balanced configuration of my loop helps to attenuate
local noise sources.
My early QSOs with ON7YD at 521 km and PA0LQ at 495 km (using my vertical on transmit)
would not have been possible without the ability to null out the Loran noise on receive.
Except when working stations at locations perpendicular to the loop, I always use
the loop on receive.
Secondly, as a transmitting antenna, the loop is easy to tune and does not involve having
to deal with the very high RF voltages encountered with loaded verticals. Also, the
tuning of the loop is largely unaffected by rain - even though the loop is very close to
our house and even though the base of the delta is only 5 ft above ground level.
My experiments with verticals are still at an early stage. Initial results on
transmit when using a 36 ft base-loaded vertical indicate a 6 dB improvement over the
delta-loop (as judged by stations at locations that are in line with the plane of the
loop). Note that the apex of the delta loop is at about the same height as the top
of the vertical. Naturally, this apparent improvement relative to the loop antenna
increases significantly for those stations at locations to the north and south of
Multi-turn Receiving Loops
Although I have never used a multi-turn receiving loop, I am told a small rotatable
loop can produce very good results. Here is a picture of the one used by Des M0AYF:
Des is grateful to the many LFers who have shared their knowledge and experience about
loop antennas. He is particularly grateful to those who have posted details of their
receiving loops on the world wide web. In particular, Des recommends the following
page which contains five high definition pictures detailing the mechanical construction of
the loop at KI0LE: